1. Field of Invention
The invention relates to an optical communication system for performing full duplex communication. More particularly, the invention relates to an optical communication system for performing full duplex communication connecting a first and a second optical transceiver units connected with each other by means of a single-core optical fiber.
2. Description of the Related Art
An optical communication system has been proposed which includes a first and a second optical transceiver units connected with each other by a single-core optical fiber to maintain full duplex communications between them. FIG. 1 illustrates such an optical communication system 200 as mentioned above.
The optical communication system 200 consists of a single-core optical fiber 210, an optical transceiver unit 220 connected to one end of the single-core optical fiber 210, and another optical transceiver unit 230 connected to the other end of the single-core optical fiber 210.
The optical transceiver unit 220 has:                a light emitting element 221 for emitting transmission light;        a drive circuit 222 for driving the light emitting element 221 in response to a transmission signal ST1;        a light receiving element 223 for receiving transmission light from light emitting element 231 of the optical transceiver unit 230;        an amplifier 224 for amplifying signal light SR1 received by the light receiving element 223; and        a guiding means 225 such as a prism for guiding the transmission light from the light emitting element 221 to the single-core optical fiber 210 and for guiding the received light from the single-core optical fiber 210 to the light receiving element 223.        
The optical transceiver unit 230 has:                the light emitting element 231 for emitting transmission light;        a drive circuit 232 for driving the light emitting element 231 in response to a transmission signal ST2;        a light receiving element 233 for receiving transmission light from light emitting element 221 of the optical transceiver unit 220;        an amplifier 234 for amplifying signal light SR2 received by the light receiving element 233; and        a guiding means 235 for guiding the transmission light from the light emitting element 231 to the single-core optical fiber 210 and for guiding the received light from the single-core optical fiber 210 to the light receiving element 233.        
Operations of the full duplex optical communication system 200 as shown in FIG. 1 are as follows.
A beam of transmission light is emitted from the light emitting element 221 of the optical transceiver unit 220 in response to the transmission signal ST1, which is guided by the guiding means 225 to one end (proximal end) of the single-core optical fiber 210 adjacent the optical transceiver unit 220. The light is further transmitted through the single-core optical fiber 210 to the other end (distal end) thereof adjacent the optical transceiver unit 230 and guided to the light receiving element 233 as the received signal light by the guiding means 235 of the optical transceiver unit 230. The light receiving element 233 provides a signal SR2 associated with the signal light received.
On the other hand, a beam of transmission light is also emitted from the light emitting element 231 of the optical transceiver unit 230 associated with a transmission signal ST2. The light is then guided by the guiding means 235 to the proximal end of the single-core optical fiber 210 adjacent the optical transceiver unit 230. The transmission light is passed through the single-core optical fiber 210 to the distal end thereof, and guided as the received signal light by the guiding means 225 to the light receiving element 223 of the optical transceiver unit 220. The light receiving element 223 provides a signal SR1 associated with the received signal light.
It is said in the example above that the light receiving element 233 of the optical transceiver unit 230 receives the signal light (referred to as optical reception signal) transmitted from the light emitting element 221 of the optical transceiver unit 220. In actuality, however, the light receiving element 233 also receives crosstalk component contained in the optical reception signal. This crosstalk results from reflection of the light emitted by the light emitting element 231 of the optical transceiver unit 230. This is also the case with the optical reception signal received by the light receiving element 223 of the optical transceiver unit 220.
Such optical crosstalk includes near-end optical crosstalk (NX-talk) and far-end optical crosstalk (FX-talk). The NX-talk results from the reflection of the incident transmission light by the proximal end of the single-core optical fiber 210 back to the very optical transceiver unit that emitted the transmission light. The FX-talk results from the reflection of the exiting transmission light at the distal end of the single-core optical fiber 210 back to that optical transceiver unit.
The optical crosstalk will be further discussed in detail with reference to FIG. 2. Of the light transmitted from the optical transceiver unit 220 to the optical transceiver unit 230, the amount of light received (i.e. detected) by the light receiving element 233 of the optical transceiver unit 230 is given by Equation (1) below.S(dBm)=TA(dBm)−La(dB/m)*D(m)−LR(dB)  (1)where TA(dBm) is the amount of light transmitted from the optical transceiver unit 220 to the single-core optical fiber 210, La (dB/m) is the amount of light dissipated in the single-core optical fiber 210 during its transmission through it, D(m) is the length of the single-core optical fiber 210, and LR(dB) is the reception loss or the amount of light that has exited the single-core optical fiber 210 but lost before entering the light receiving element 233.
Of the transmission light emitted from the optical transceiver unit 230 to the optical transceiver unit 220, the amount of FX-talk, i.e. the transmission light reflected at the distal end back to the light receiving element 233 of the optical transceiver unit 230, is given by Equation (2) below.
                                                                        FX                ⁡                                  (                  dBm                  )                                            =                            ⁢                                                TB                  ⁡                                      (                    dBm                    )                                                  -                                                      Lb                    ⁡                                          (                                              dB                        ⁢                                                  /                                                ⁢                        m                                            )                                                        *                                      D                    ⁡                                          (                      m                      )                                                                      -                                  LF                  ⁡                                      (                    dB                    )                                                  -                                                      Lb                    ⁡                                          (                                              dB                        ⁢                                                  /                                                ⁢                        m                                            )                                                        *                                                                                                                      ⁢                                                D                  ⁡                                      (                    m                    )                                                  -                                  LR                  ⁡                                      (                    dB                    )                                                                                                                          =                            ⁢                                                TB                  ⁡                                      (                    dBm                    )                                                  -                                  2                  *                                      Lb                    ⁡                                          (                                              dB                        ⁢                                                  /                                                ⁢                        m                                            )                                                        *                                      D                    ⁡                                          (                      m                      )                                                                      -                                  LF                  ⁡                                      (                    dB                    )                                                  -                                  LR                  ⁡                                      (                    dB                    )                                                                                                          (        2        )            where TB(dBm) is the amount of light emitted from the optical transceiver unit 230 and coupled with the single-core optical fiber 210, Lb(dB/m) is the amount of light lost during the transmission through the single-core optical fiber 210, and LF(dB) is the amount of light reflected at the distal end of the single-core optical fiber 210, resulting in the FX-talk.
The amount of NX-talk detected by the light receiving element 233 of the optical transceiver unit 230, i.e. the light emitted from the optical transceiver unit 230 to the optical transceiver unit 220 but reflected at the proximate end of the single-core optical fiber 210, is given by Equation (3) below.NX(dBm)=TB(dBm) −LN(dB)  (3)where LN(dB) is the amount of light reflected at the proximal end of the single-core optical fiber 210 backward and lost before detected by the light receiving element 233.
The total amount X of crosstalk component detected by the light receiving element 233 of the optical transceiver unit 230 is obtained from Equations (2) and (3) as
                                                                        X                ⁡                                  (                  dBm                  )                                            =                            ⁢                              10                *                                  LOG                  ⁡                                      (                                                                  10                        ⋀                                                  (                                                                                    FX                              ⁡                                                              (                                dBm                                )                                                                                      /                            10                                                    )                                                                    +                                              10                        ⋀                                                  (                                                                                    NX                              ⁡                                                              (                                dBm                                )                                                                                      /                            10                                                    )                                                                                      )                                                                                                                          =                            ⁢                              10                *                                  LOG                  (                                                            10                      ⋀                                              (                                                                              (                                                          TB                              -                                                              2                                *                                Lb                                *                                D                                                            -                              LF                              -                              LR                                                        )                                                    ⁢                          10                                                )                                                              +                                                                                                                                        ⁢                                                10                  ⋀                                      (                                                                  (                                                  TB                          -                          LN                                                )                                            /                      10                                        )                                                  )                                                                        (        4        )            
It is noted that when the light loss La (dB/m) is larger than the light loss Lb (dB/m), the amount of light received, S (dBm), as given by Equation (1) decreases relatively, so that the amount of the crosstalk X as determined by Equation (4) increases relatively.
As an example, light loss La (dB/m) can be larger than light loss Lb (dB/m) when the power spectra of the light emitting elements 221 and 231 have different spectral widths in wavelength. For example, given a single-core optical fiber 210 characterized by a low transmission loss window over a wavelength band as shown in FIG. 3C, the width of the power spectrum of the light emitting element 221 is wider than the window as shown in FIG. 3A while the width of the power spectrum of the light emitting element 231 is narrower than the window as shown in FIG. 3B. This can happen when a light emitting diode is used for the light emitting element 221 and a laser diode is used for the light emitting element 231.
The light loss La (dB/m) can be larger than the light loss Lb (dB/m) also in the event that the peak wavelength of the light emitted from the light emitting element 221 varies greatly as compared with that of the light emitted from the light emitting element 231. For example, given a single-core optical fiber 210 having a low transmission loss spectrum as shown in FIG. 4, the peak wavelength of light emitted from the light emitting element 221 greatly varies with temperature as shown in FIG. 4A, while the wavelength of light emitted from the light emitting element 231 is not so as shown in FIG. 4B.
As another example, light loss La (dB/m) can be larger than light loss Lb (dB/m) when the wavelengths of light emitted by the light emitting elements 221 and 231 are not the same. This is also the case when the single-core optical fiber 210 has a transmission loss spectrum over a range as shown in FIG. 5C, the (peak) wavelength of the light emitted from the light emitting element 221 is short (e.g. at or near the lower end of the range) as shown in FIG. 5A, while the wavelength of the light emitted from the light emitting element 231 is long (e.g. at or near the upper end of the range) as shown in FIG. 5B.
As a further example, light loss La (dB/m) can be larger than light loss Lb (dB/m) when the launched numerical apertures (LNA) for the incident light emitted from the light emitting elements 221 and 231 to the single-core optical fiber 210 are different. For example, LNA (=sin α) for the light emitting element 221 as shown in FIG. 6A is smaller than that for the light emitting element 231 as shown in FIG. 6B.
As described above, the signal components fed to the respective light receiving elements 223 and 233 of the respective optical transceiver units 220 and 230 are weakened by the crosstalk components contained in the signal light. In addition, the signal components are influenced by a Gaussian noise. Hence, the aperture phase margin therefor is appreciably reduced.
Referring now to FIGS. 7A-9B, harmful influences of optical crosstalk on the reception eye pattern of a signal received will be discussed.
FIGS. 7A and 7B show reception eye patterns not influenced by any optical crosstalk component. In this instance, received signal light supplied to the light receiving elements contain only signal components, as seen in FIG. 7A. Thus, the reception eye patterns are determined solely by the signal components, as shown in FIG. 7B.
FIGS. 8A and 8B show reception eye patterns slightly influenced by optical crosstalk. In this instance, received signal light supplied to the respective light receiving elements contain appreciable amounts of optical crosstalk component, as shown in FIG. 8A. Hence, the reception eye pattern of a received signal has a high level (H) and a low level (L) slightly offset from the levels shown in FIG. 8a by the crosstalk component, as shown in FIG. 8B.
FIGS. 9A and 9B show an optical communication system involving a significant amount of optical crosstalk. In this instance, the light supplied to each of the light receiving elements contains significant crosstalk component as seen in FIG. 9A. Consequently, the high level (H) and the low level (L) of the reception eye pattern, are significantly offset by the crosstalk component, as shown in FIG. 9B.
In addition, a Gaussian noise is superposed on each of the high level H and the low level L of the reception eye pattern, as shown in FIGS. 7B, 8B, and 9B. The Gaussian noise is the root mean square (rms) of different types of noise generated by the circuit elements involved including the light emitting elements and amplifiers.
A bit error rate (BER) of the signal received is conventionally determined by Equation (5) below, based on an assumption that both the high level H and the low level L of the reception eye pattern are superposed with the Gaussian noise,
                                                        BER              =                            ⁢                                                                    1                    2                                    ⁢                                                            ∫                                              -                        ∞                                            D                                        ⁢                                                                  1                                                  σ                          ⁢                                                                                    2                              ⁢                              π                                                                                                                          ⁢                                              exp                        (                                                  -                                                                                                                    (                                                                  H                                  -                                  x                                                                )                                                            2                                                                                      2                              ⁢                                                              σ                                2                                                                                                                                    )                                            ⁢                                                                                          ⁢                                              ⅆ                        x                                                                                            +                                                                                                      ⁢                                                1                  2                                ⁢                                                      ∫                    D                    ∞                                    ⁢                                                            1                                              σ                        ⁢                                                                              2                            ⁢                            π                                                                                                                ⁢                                          exp                      (                                              -                                                                                                            (                                                              x                                -                                L                                                            )                                                        2                                                                                2                            ⁢                                                          σ                              2                                                                                                                          )                                        ⁢                                                                                  ⁢                                          ⅆ                      x                                                                                                                                              =                            ⁢                                                ∫                  D                  ∞                                ⁢                                                      1                                          σ                      ⁢                                                                        2                          ⁢                          π                                                                                                      ⁢                                      exp                    (                                          -                                                                                                    (                                                          x                              -                              L                                                        )                                                    2                                                                          2                          ⁢                                                      σ                            2                                                                                                                )                                    ⁢                                                                          ⁢                                      ⅆ                    x                                                                                                                          =                            ⁢                                                ∫                                                            H                      -                      L                                                              2                      ⁢                      σ                                                        ∞                                ⁢                                                      1                                                                  2                        ⁢                        π                                                                              ⁢                                      exp                    (                                          -                                                                        y                          2                                                2                                                              )                                    ⁢                                                                          ⁢                                      ⅆ                    y                                                                                                                          =                            ⁢                                                1                                                            2                      ⁢                      π                                                                      ⁢                                                      ∫                    Q                    ∞                                    ⁢                                                            exp                      ⁡                                              (                                                  -                                                                                    y                              2                                                        2                                                                          )                                                              ⁢                                                                                  ⁢                                          ⅆ                      y                                                                                                                              (        5        )            where y and Q are defined by the following formula.
  y  =                              x          -          L                σ            ⁢                          ⁢      and      ⁢                          ⁢      Q        =                  H        -        L                    2        ⁢        σ            
For simplicity, it is assumed in Equation (5) that the standard deviations σ of the Gaussian noise superposed on the high level H and the low level L are the same. In Equation (5), the bit error rate is defined to be the ratio of the overlapping area of two Gaussian noise distributions appearing at the high and lower levels, to the entire area occupied by the two distributions.
As discussed above, the larger the optical crosstalk, the lager are the offset of the H and L levels. Consequently, the aperture phase margin for the bit error rate smaller than 10−α (α=12 for example) decreases with the amount of the crosstalk component.
It is therefore an object of the present invention to provide an optical communication system enabling stable full duplex communication with relatively reduced optical crosstalk, without requiring large-scale or costly transceiver units.